1. Field of the Invention
The present invention relates generally to a shock absorber and vibration dampening device for a blast hole drilling machine, and more specifically to a shock absorbing assembly for absorbing axial and torsional forces during the operation of drilling machines employing down-hole hammers.
2. Description of the Prior Art
In various types of drilling operations, the drill bit is forced downward under pressure while being rotated in order to penetrate earthen formations. These drilling operations can require the application of relatively high downward force to the drill bit as well as relatively high torque.
One example is the typical rotary blasthole drill which comprises a large drilling rig to which is attached a rotary drive mechanism. Typically, the drill's rotary drive is capable of being raised and lowered along a substantially vertical axis directly above the formation to be drilled. Additionally, a length of drill pipe or drill string is connected to the rotary drive so as to extend downwardly therefrom in a substantially vertical direction. A drill bit is secured to the downward end of the drill pipe. The drill machine's rotary drive head is activated to rotate both the drill pipe and the drill bit at the desired speed. Then, the rotary drive, together with the drill pipe and bit, is lowered so that the drill bit contacts the surface of the formation to be drilled. Downward pressure is then continuously applied to the rotating drill pipe and bit to force the drill bit to cut downwardly into the formation. As the drilling operation occurs, air is forced through the interior of the drive head, drill pipe, and through the drill bit, thereby forcing cuttings out of the hole and maintaining a clear surface upon which the drill bit may operate.
As soon as the drilled hole is deep enough to accommodate the first length of drill pipe, the drill's rotary drive is disconnected from the drill pipe and raised to its original position. A second length of drill pipe is then connected between the rotary drive and the first length of drill pipe. The rotary drive is then again activated and drilling operations are continued. This procedure is repeated until a desired hole depth is achieved.
In order to eliminate the problems associated with vibration and shock to the drilling apparatus, various devices have been employed to dampen vibrations and absorb shocks during the operation of the rotary drill. These devices typically comprise a shock absorbing apparatus which is connected between the drill machine's rotary drive head and the drill pipe. In some instances, the shock absorbing device includes some type of resilient material which absorbs the vibrations and shocks, and dissipates the undesirable energy associated with the drilling operation.
U.S. Pat. Nos. 3,746,330 and 3,947,009 show a resilient coupling provided between a drive shaft and a driven shaft of a tubular drill string. A group of resilient discs are sandwiched between a series of axially-spaced drive, driven and pressure plates which surround drive and driven shaft members. Pin projections extend from the driven plate into the resilient discs while fastening means extend between the drive and pressure plates for compressing the resilient discs together and into union with the pin projections and fastening means.
U.S. Pat. No. 4,109,488 shows a shock absorbing rotary drive coupling for a rotary blasthole drill. The device includes two parallel, horizontal plates. One of the plates is connected to the rotary drive and the other is connected to the drill pipe. The apparatus further includes a resilient pad which is bonded between the two plates. The entire apparatus has a hole through its center in order to accommodate the air and fluid which is forced through the drill pipe to the drill bit.
In certain of the prior art devices, axial vibrations were absorbed by forcing the drive and driven plates closer together. For instance, the old "DRILCO" shock absorber featured an internal piston action. U.S. Pat. No. 4,571,215, issued Feb. 18, 1986, is similar in that the drive and driven plates are forced closer together. In these and similar designs, the resilient pad between the two plates was compressed, causing the resilient pad to dissipate the energy of vibration as heat. Other of the prior art devices failed to adequately respond to torsional, i.e. rotational vibrations at all and, in fact, were restricted by solid members. When subjected to torsional vibration, a shearing force was exerted on the resilient pad. Over a period of time or in response to large torsional stresses, the resilient pad was frequently broken. As a result, these devices tended to wear out within a relatively short time, thereby necessitating frequent repair or replacement. This factor substantially increased the cost of the drilling operation.
An aspect of the present invention relates to the use and effect of a tensed and stretched elastomer as a resilient member for absorbing and dampening forces. In an article entitled "Heat Build-Up of Dynamically-Loaded Engineered Elastomeric Components" that was presented at Designing With Elastomers Educational Symposium, Energy Rubber Group, Houston, Tex., Sep. 24- 26, 1990, it was stated that "[t]he apparent modulus of an elastomer component in tension or compression (shear is unaffected) is a function of the inherent dynamic properties of the elastomer and a geometric design variable . . ." The article completely fails to suggest that tension has any advantages over compression. In a second article entitled "Engineering Design With Natural Rubber," published by The Malaysian Rubber Producers' Research Association (1978), it is stated that "[t]he load-deflexion curves for rubber in tension and compression are approximately linear." The articles goes on to state that "the values of Young's modulus in tension and compression are approximately equal." This would suggest that the shock absorption characteristics of rubber in tension would not provide better results than compression. The inventors have found that contrary to present understanding, the compression of elastomeric resilient materials makes the materials stiffen. The compressed resilient material, therefore, transmits some of the longitudinal impact forces and torsional forces making them less effective while in compression. Also, the compression of the resilient material makes them less effective for absorbing impact and torsional forces; their use as a shock absorbing and vibration dampening material is somewhat limited (shock absorption, as used herein, also includes vibration dampening). As explained below, an elastic material for absorbing shock through tension of the material is much more effective in the present invention than using compression of the elastic material.
It is the object of the present invention to provide a shock absorbing/vibration dampening assembly for a drilling machine which is simple and economical in design and which effectively dampens both rotational torque and axial vibrations encountered during down-hole hammer drilling operations.
Another object of the invention is to provide such a shock absorbing assembly wherein there is virtually no possibility of breaking the resilient member of the assembly.
It is also an object of the invention to provide a shock absorbing vibration dampening assembly capable of transmitting the downward pressures required on hammer-type drill bits downwardly into the formation to be drilled while simultaneously transmitting the torsional forces required for rotating the bit and while absorbing the shock and vibration of the air-operated hammer encountered during the down-hole hammer drilling operation.